1. Field of the Invention
The present invention relates to a bias material for magnetic markers for applying a bias magnetic field to a metal piece (hereafter called a magnetostriction element) which oscillates under the magnetostriction effect, a magnetic marker, and a method of producing a bias material for magnetic markers.
2. Description of Related Art
Some electronic monitoring systems have been proposed, by which magnetic labels attached to commodities are detected as markers for a purpose of preventing theft or watching a transferring flow or kinds of commodities, goods or articles. Among such systems, there is a system in which markers are made from a magnetostriction material.
For example, JP-A-8-60312 (EP702096A1) teaches to use an amorphous magnetostriction material as a marker. Particularly, JP'312 discloses a system according to which a magnetostriction element made of a specific amorphous metal is detected by a pickup coil by way of causing the magnetostriction element to resonate within an alternating magnetic field so as to vary the magnetic field.
WO-96/32518 (USP5628840) and JP-A-8-87237 teach, respectively, a magnetic marker which is a combination of a magnetostriction material and a bias material which exerts a preliminary magnetic field (or a bias magnetic field) on the magnetostriction material. The magnetic marker taught in the publications is preliminarily provided with a bias magnetic field so that the magnetostriction material mechanically resonates under exertion of an alternating magnetic field.
According to such magnetic marker in which the bias material is made of a semi-hard magnetic material by which a magnetostriction element for a marker is exposed to a bias magnetic field, the marker can be inactivated by demagnetizing the bias material without removing it from a commodity or article.
Thus, if markers are inserted into or attached to commodities or articles, they can be distinguished from one another by way of inactivating the magnetic markers being attached to the commodities when paying therefor, which are those properly purchased and those intended to illegally bring out from a shop, the latter commodities carrying active magnetic markers.
In such electronic monitoring systems, a selection of the bias material for applying a bias magnetic field to the magnetostriction material is important as well as a selection of the magnetostriction material for markers.
The bias material is required to have a higher coercive force than the magnetostriction material so as not to be demagnetized by the magnetostriction material, and must be magnetizable and demagnetizable.
As magnetizable and demagnetizable materials, there have been known Fe--Cr--Co semi-hard magnetic materials represented by an Fe--25Cr--10Co alloy (by weight percent), which are used for lead relays and other applications.
Although Fe--Cr--Co alloys have been used also as bias materials, they are expensive because of a large amount of Co, which have coercive force of as high as 7,200 A/m, residual magnetic flux density (Br) of about 1.1 T, and remanence ratio (Br/B8,000), which is a ratio of magnetic flux density at an imparted magnetic field of 8,000 A/m (saturated magnetic flux density (B8,000)) to residual magnetic flux density (Br), is as relatively low as about 0.8.
As mentioned above, the bias material is a material which can be magnetized and demagnetized, and if its coercive force is too high, it is difficult to demagnetize. If the bias material is not fully demagnetized, the electronic monitoring system may be operated erroneously.
Similarly, if the remanence ratio and magnetization steepness are unsatisfactory in the B-H curve, the boundary between magnetized and demagnetized states is unclear also resulting in erroneous operation.
The term "magnetization steepness" (or steep rise and fall magnetization property) used herein is the property of radical or abrupt change in the magnetized state when the bias material is magnetized or demagnetized, and the B-H curve of materials which is significant or excellent in magnetization steepness have a generally rectangular form.
It is determined as follows whether or not the magnetization steepness is significant or excellent.
In general, it has been believed that a semi-hard magnetic material reaches a saturated magnetic flux density when a magnetic field of five times the coercive force of the material is applied. This magnetic flux density is herein called B(5Hc). Further, a magnetic flux density when a magnetic field of 1.5 times the coercive force of the material is applied is called B(1.5Hc), and when the ratio of B(1.5Hc)/B(5Hc) exceeds 0.8, the material is herein determined to have excellent magnetization steepness.
While the residual magnetic flux density is preferably as high as possible for applying a bias magnetic field to the magnetostriction element, that of Fe--Cr--Co alloys is much lower than 2.1T of residual magnetic flux density of pure iron.
The magnitude of the bias magnetic field applied to the magnetostriction element is in proportion to the residual magnetic flux density and the cross-sectional area of the bias material. Also, if the residual magnetic flux density is low, the cross-sectional area of the bias material must be increased, which is not suitable for downsizing the magnetic markers.